.\"	$OpenBSD: listen.2,v 1.2 1996/03/19 23:15:16 niklas Exp $
.\"	$NetBSD: listen.2,v 1.7 1996/02/16 20:38:45 phil Exp $
.\"
.\" Copyright (c) 1983, 1991, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
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.\" modification, are permitted provided that the following conditions
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.\" 3. All advertising materials mentioning features or use of this software
.\"    must display the following acknowledgement:
.\"	This product includes software developed by the University of
.\"	California, Berkeley and its contributors.
.\" 4. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
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.\"     @(#)listen.2	8.2 (Berkeley) 12/11/93
.\"
.Dd March 18, 2015
.Dt LISTEN 2
.Os BSD 4.2
.Sh NAME
.Nm listen
.Nd listen for connections on a socket
.Sh SYNOPSIS
.Fd #include <sys/socket.h>
.Ft int
.Fo listen
.Fa "int socket"
.Fa "int backlog"
.Fc
.Sh DESCRIPTION
Creation of socket-based connections requires several operations.
First, a socket is created with
.Xr socket 2 .
Next, a willingness to accept incoming connections and
a queue limit for incoming connections are specified with
.Fn listen .
Finally, the connections are
accepted with
.Xr accept 2 .
The
.Fn listen
call applies only to sockets of type
.Dv SOCK_STREAM.
.\"or
.\".Dv SOCK_SEQPACKET.
.Pp
The
.Fa backlog
parameter defines the maximum length
for the queue of pending connections.
If a connection
request arrives with the queue full,
the client may receive an error with an indication of
.Er ECONNREFUSED .
Alternatively, if the underlying protocol supports retransmission,
the request may be ignored so that retries may succeed.
.Sh RETURN VALUES
.Rv -std listen
.Sh ERRORS
.Fn listen
will fail if:
.Bl -tag -width Er
.\" ==========
.It Bq Er EACCES
The current process has insufficient privileges.
.\" ==========
.It Bq Er EBADF
The argument
.Fa socket
is not a valid file descriptor.
.\" ==========
.It Bq Er EDESTADDRREQ
The socket is not bound to a local address
and the protocol does not support listening on an unbound socket.
.\" ==========
.It Bq Er EINVAL
.Fa socket
is already connected.
.\" ==========
.It Bq Er ENOTSOCK
The argument
.Fa socket
does not reference a socket.
.\" ==========
.It Bq Er EOPNOTSUPP
The socket is not of a type that supports the operation
.Fn listen .
.El
.Sh SEE ALSO
.Xr accept 2 ,
.Xr connect 2 ,
.Xr connectx 2 ,
.Xr socket 2
.Sh BUGS
The
.Fa backlog
is currently limited (silently) to 128.
.Sh HISTORY
The
.Fn listen
function call appeared in
.Bx 4.2 .
